docs: def sheets(self):
          """
          Collection of sheets in this document.
          """
          # http://www.openoffice.org/api/docs/common/ref/com/sun/star/sheet/XSpreadsheetDocument.html#getSheets
          try:
              return self._sheets
          except AttributeError:
              target = self._target.getSheets()
              self._sheets = SpreadsheetCollection(self, target)
          return self._sheets